The present invention relates to packages for high-speed integrated circuits, and more particularly relates to the transmission structures for connecting to high-speed integrated circuit chips for optical/electronic and wired/wireless communications.
In optical/electronic and wired/wireless communications, it is increasingly common to communicate using signals with frequencies well into the ranges of a few GHz or tens of GHz. For example, for OC-192/STM-64 optical transmission, the frequency range may be 9 GHz to 14 GHz. For OC-768/STM-256 optical transmission, the frequency range may be, for instance, from 20 GHz to 50 GHz. For the third-generation cellular technology, the frequency range of interest may be between 1.885 GHz and 2.2 GHz and even into 5 GHz with the 802.11 standard. As a result, integrated circuits (xe2x80x9cICsxe2x80x9d) suited for these high-speed applications are now becoming more in demand than before.
Before these high-speed ICs can be placed onto a printed wiring board (xe2x80x9cPWBxe2x80x9d) or printed circuit board (xe2x80x9cPCBxe2x80x9d), they need to be packaged either as a single chip, or as a multi-chip module. In addition to providing ease of handling and installation, the primary function of a package is one of dimensional transformation. While at the chip level, the input/output (xe2x80x9cI/Oxe2x80x9d) pad size and spacing are in the order of approximately 3 to 5 mils, the same dimensions at the PWB level are typically 10 to 40 mils. At frequencies below 1 GHz, fanning out using short transmission lines can generally accomplish this objective. As the operating frequency of the chip approaches 10 GHz or higher, the task of dimensional transformation needs to be accomplished, while maintaining the characteristic impedance of the overall transmission pathway.
FIG. 1 illustrates a simplified diagram for an exemplary conventional device package. This exemplary package 100 may be a demultiplexing (xe2x80x9cdemuxxe2x80x9d) device for optical communication, where an input data signal is demultiplexed into multiple lower-speed data signals. This package typically contains a single-layer Alumina (Al2O3) substrate 110 with a typical thickness of 10 mils, an integrated circuit die 120 residing in a recess 130 formed on the surface of the substrate, and transmission structures 140 on the substrate. The transmission structures, commonly called microstrips, behave preferably like 50-ohm signal transmission lines suitable for high-speed devices. The microstrips are wire-bonded 135 to the signals pads 136 on the die 120, and connected to the lead terminals 150 of the package. For high-speed signals, such as clocks and high-speed data signals to a demux device, the microstrip dimensions are further restrained by how they are connected to the external I/O terminals 150. High-speed signals in the ranges of 20-50 GHz may need to be connected using small coaxial connectors, such as the GPPO connectors manufactured by Gilbert Electronics. These coaxial connectors typically have a 30 mil overall diameter with a 10 mil conductor core. A good microwave transition occurs if the diameter of the conductor core is compatible with the width of the microstrip and the thickness of the substrate 110 matches with the width of the annular ring dieletric ring in the connector. This matching takes place if, for example, a substrate thickness of 10 mils and a microstrip width of 10 mils are selected. On Alumina such a microstrip line has a characteristic impedance of 50 ohms. However, a microstrip of this width cannot be supported at the die side, due to the dimension and location requirements of the signal pads on the die.
To maintain the 50-ohm characteristic impedance, the width of each of the transmission structures 140 needs to be around 10 mils wide for a 10-mil thick Alumina substrate, and the spacing between the transmission structures 140 needs to be at least 10 mils in order to maintain 10% impedance accuracy and minimize cross talk. The constraints on width and spacing of the transmission structures 140 on the substrate limit the density of the signal pads 136 on the die. In this instance, the signal pads cannot be placed closer than 20 mils center-to-center. For low speed or low I/O-count devices, the spacing is typically not a problem because one either does not need to have 50 ohms lines on the package leading to the die or the low I/O count allows sparse spacing of the pads on the die to accommodate the spacing of transmission lines on the package.
For high speed and high I/O-count dies, such a wide pad spacing as dictated by the package is clearly not acceptable. For example, a demux device, such as the one used for OC-768/STM-256 transmissions, may support 16 data signals, or 32 signals when differential signals are utilized. Each of the 32 signals requires a bonding wire connection from the die""s signal pad to the substrate and a microstrip connection from the substrate to the lead terminals at the perimeter of the package. As can be appreciated by those skilled in the art, in order to minimize cost, die size is always kept to a minimum thus resulting in limited perimeter allocated for signal pads and the necessary separation between them. Generally it would be desirable to have signal pads that have 3-5 mils center-to-center spacing. If the microstrip is too wide (e.g., 20 mils center-to-center), it encroaches upon its neighboring microstrips, thus forcing others out of alignment with their corresponding signal pads on the die.
Further, the connecting structures for the high-speed signals need to maintain a 50-ohm impedance continuity from the connector to the pad. However, when wire bonds are applied to connect the signal pads to their corresponding microstrips, a discontinuity is created that disrupts the 50-ohm impedance environment, due to the parasitic capacitance in the pads and the parasitic inductance in the wire bonds themselves.
Therefore, while it is desirable to be able to dimensionally transform the width of the high-speed transmission lines from the die side to the package connector side, it is also desirable to be able to maintain the 50-ohm impedance continuity for the connecting structure at all points for the very high frequencies.
The same challenges also confront another version of the connection package, known as Ball Grid Array (xe2x80x9cBGAxe2x80x9d). BGAs are packages with I/Os dispersed in a two-dimensional rectangular array of solder balls. Such a package is then mounted to the printed wiring board by a solder reflow process. One of the advantages of a BGA package is that with a 2-dimensional I/O array, it can accommodate a large number of I/O signals. However, a BGA package also requires some, if not all, signals to pass through a multi-layer substrate, thus complicating the task of dimensional transformation and impedance continuity.
Methods and apparatus for packaging high-speed integrated circuits in optical, electronic, wired or wireless communications are disclosed. The connection package achieves dimensional transformation of signal routes from the IC""s input/output pads to the external terminals such as coaxial terminals and BGA balls, while maintaining constant characteristic impedance throughout the transmission lines. In accordance with one embodiment of the present invention, the connection package includes a substrate for positioning the IC, microstrips for communicating from the IC""s pads to the external terminals. At least a pair of the microstrips, such as the differential signals of the IC, can be positioned closer to each other so that a portion of the differential pair becomes capacitively coupled. Such coupled capacitance allows the width of the differential microstrips to be reduced so that they can accommodate the spacing requirements imposed by the signal pads on the IC. Further, to achieve impedance continuity, a portion of the coupled microstrips near the die is widened to increase the capacitance so that the overall transmission path can become an all-pass networkxe2x80x94from the signal pad, through the bonding wires, to the microstrip. For the rest of the portions of the microstrips, they can be tapered and fan out to their respective external connectors.
In accordance with another embodiment of the present invention, a connection package for high-speed integrated circuits with a multi-layer substrate is disclosed. The multi-layer package includes a substrate, at least one coaxial external terminal for communicating a high-speed signal to or from the IC, an array of terminals for communicating lower-speed signals to or from the IC, microstrips formed on the substrate to connect the high-speed signal to the coaxial terminal, microstrips formed on the substrate to connect the lower-speed signals to the array of terminals, and inter-layer connections formed within the substrate. The microstrip for connecting the high-speed signal to the coaxial terminal can have its dimension successfully transformed by controlling its space, and hence the capacitance, to ground planes of the multi-layered substrate. Continuity in the transmission paths can be maintained by maintaining constant characteristic impedance for each segment of the microstrips, inter-layer connections and external terminals.